I honestly think that you don't understand the question
I don't understand your question either, and it's probably because I believe you're operating under incorrect assumptions.
Let me be clear - your provider doesn't care about you trying to NAT traverse to make your Skype calls better. They care about making money, minimizing outages, and lowering their transit costs if they're not tier 1. That's basically it. The "large scale" or "carrier grade" routers you're referring to have very little to nothing to do with NAT or "picking ports". Even routers that are directly upstream of the router in your home (or the the Winn Dixie), don't care about NAT. The whole point of NAT is that your end device doesn't know it's being NATed, and the carrier's edge router doesn't care - they see a destination IP address and a source IP address which they have allocated to you, and they want to get those packets off of their network as quickly as possible. As Ron has said, NAT was designed for address preservation/conservation and "mobility" in mind, so that customers at the edge of the carrier's network could make the most use out of their allocated address space. Most routers at the "large" scales you're referring to are not deployed in production networks with design considerations that involve doing any kind of NAT, they're implemented for their FIB/RIB capacity, port density, type of interfaces available, TCAM size, backplane speed, etc. etc. The finger pointing regarding the port selection need not be at the carrier and their equipment, it needs to be squarely at the client initiating the connection and its upstream first-hop L3 device that's performing the NAT.
So if your question is, "why don't the clients pick easy to guess ports?" (or say sequential ports), the answer is many implementations had done just this when choosing an ephemeral port: "take the last currently in-use port, add one, get new ephemeral port"
However, you need to understand that choosing an ephemeral port isn't only something that the client has to care about - a "collision" would occur in the following scenario (somewhat simplified and contrived):
CLIENT_IP:EPH_PORT <----FIN---- WEBSERVER:80
CLIENT_IP:EPH_PORT --FIN/ACK--> WEBSERVER:80
CLIENT_IP:EPH_PORT <----ACK---- WEBSERVER:80
So now the client's state of the connection is gone, and the webserver's TCP session is in
FIN_WAIT. If the client used the "next available + 1" ephemeral port criteria, and used the same ephemeral port that it used previously to connect to the same webserver while the previous session was still in
FIN_WAIT on the webserver, there would be a problem.
Regarding the blocking of everything outbound but 80 and 443, that's again up to the person that manages the equipment at the edge, not the carrier. Carriers simply just don't do this. If I'm feeling like an extremely cautious or paranoid admin and I was managing the network infrastructure/WiFi in a highly public place, I would likely do the same thing.
NAT is not a means of security. The ephemeral ports chosen for NAT can be and sometimes are obfuscated for security measures however, as you've noticed; an attacker can in fact do nefarious things if they have a means of easily guessing the next ephemeral port used in a transport-layer flow.